The Latics boss was left to rue missed chances on Saturday as he saw his side fall to a 2-1 home defeat to Fulham, with former striker and Wigan's all-time English Premier League leading goalscorer, Hugo Rodallega, opening his account for the opposition.
As a result, Martinez has thrown open the competition for places for this weekend's trip to
"We want to win every single game, not just in the last third of the season," Martinez said.
"We need to improve a little bit more across the board and our performances need to be more solid.
"Obviously there'll be positions up for grabs against Sunderland."
Martinez did have some positives to take from his side's defeat at the
Miyaichi, 19, and McManaman, 21, came on as replacements for defensive duo
The young duo impressed the Wigan boss, putting them in pole position to be handed more starts.
"That's a real positive for us to have a couple of youngsters of that quality," Martinez said.
"I did think they were influential (against Fulham), the way that they came on and were infectious, they played with a real enjoyment and probably we lacked that rhythm early on in the game.
"So I was really pleased with that and I think it shows the competition we've got now for places.
"It's so important that everyone's got an opportunity. It's great to see Callum's development over the last two or three seasons and I think he showed that he's ready to help the team in many games to come.
"Ryo slowly is adapting and getting ready, and when he's at his best he can be a player that gives you penetration and makes things happen."
Martinez also admitted he believes his side has lost some momentum during the recent international break, creating areas that need to be focused on if it is to regain its form.
Wigan played well against European champion Chelsea despite its opening-day defeat before good wins over Southampton and against Nottingham Forest in the cup saw it off to a good start.
However, its form has since dipped, with a battling draw against Stoke before the international break its most recent point, ahead of back-to-back defeats by Manchester United and Fulham.
Martinez said: "(On Saturday) in certain things we weren't as good as we can be. Some aspects we did very well and in others we were very poor.
"I was delighted with the manner we started the season, which was exactly the same as we finished the last campaign.
"It seems since we had the international break probably we've lost a little bit of focus and certain relationships on the pitch have become a little bit soft and we need to react and address that."
The future of Wigan manager